This invention provides a radiation source which can emit X-ray flux and other forms of radiation producible by an electron beam current. The substance of the invention is the formation of the cathode or cathode array which produces the electron beam current on the window through which the radiation will exit the source. The radiation source disclosed herein can be made in formats which are compact or flat as compared with prior art radiation sources. X-ray flux produced by the invention can be used for such purposes as radiation imaging, sterilization, decontamination of biohazards or photolithography.
Radiation has come to be used for many purposes. Since the discovery of X-radiation by Roentgen and others over 100 years ago, X-rays have found widespread use in medical, industrial and scientific imaging as well as in sterilization, lithography, medical radiation therapies and a variety of scientific instruments. X-rays are most commonly produced with vacuum X-rays tubes, the operation of which is shown conceptually in FIG. 1a and in diagram in FIG. 2. An electron beam source, traditionally a hot filament cathode, is biased at a high potential across a vacuum relative to a metal anode which serves as an X-ray target. Current from the cathode produces both characteristic line radiation and Bremsstrahlung radiation as it strikes the anode target. The target is commonly disposed at an angle to the electron beam current so as to direct the X-rays thus produced out a window, this window commonly being made of a material, such as beryllium, with a low atomic number (Z number). As a general matter, the higher the Z number of the target, and the higher the electrical potential and energy of the beam, the more X-radiation is produced. The lower the Z number of the window, the less radiation is absorbed by the window. Radiation which does not exit the window is absorbed elsewhere in the tube. X-ray flux may be collimated by limiting the flux which exits to tube to a small window. X-ray tubes commonly have low power efficiencies; typically only about 1% of the power used to produce the electron beam current is realized in the X-ray beam energy exiting the tube. The production of X-rays by the electron beam striking the target also generates a considerable amount of heat, since most of the beam energy is absorbed in the target. Numerous inventions have been made over the years to conduct this heat out of the tube, to improve the X-ray production efficiency of the target, or to rotate the anode so as to reduce pitting or melting of the target. (J. Selman. The Fundamentals of X-Ray and Radium Physics, 8, ed. Thomas Books Springfield, Ill. 1994).
Recently, a number of inventions have been made in which the traditional hot filament cathode in an X-ray tube is replaced with a cold cathode operating on the principles of field emission. Field emission cold cathodes have a number of advantages over hot filament cathodes. They do not require a separate heater to generate an electron beam current, so they consume less power. They can be turned on and off instantly in comparison with filament cathodes. They can also be made very small, so as to be used in miniature X-ray sources for radiation therapy, for example. U.S. Pat. Nos. 5,854,822 and 6,477,233 disclose examples of miniature cold cathode X-ray tubes. U.S. Pat. Nos. 6,760,407 and 6,876,724 disclose examples of larger X-ray tubes using cold cathodes for other purposes, such as imaging. Several types of field emission cold cathodes have been developed which can be substituted for hot filament cathodes. These include arrays of semiconductor or metal microtips, flat cathodes of low work function materials and arrays of carbon or other nanotubes. While they offer several improvements, these cold cathode X-ray tubes share the limitations of their hot filament tube predecessors in being essentially point sources of X-rays. U.S. Pat. No. 6,333,968 discloses a transmission cathode for X-ray production in which current from the cathode generates X-rays on a target opposite the cathode, the radiation then transmitting through the cathode. The cathode covers substantially the entire exit area for the radiation. This limits the size of the radiation exit area to the size of the cathode, making this type of source essentially a point source of X-rays.
Other recent inventions have been made which use a wide area cold cathode or cold cathode array opposite a thin-film X-ray target disposed on an exit window. Examples are disclosed in U.S. Pat. Nos. 6,477,233 and 6,674,837. In these X-ray sources, the wide-area or pixelated beam of electrons produces a wide-area or pixelated source of X-rays. Electrons striking the X-ray target produce X-radiation in all directions. As shown conceptually in FIG. 1b, if the target is made thin enough, a portion of the X-rays will exit the side of the target opposite the electron beam source and pass through the exit window. A limitation of this type of X-ray source is that the heat produced in this process can be difficult to manage. The thinner the target film, the more X-ray flux can pass through the exit side, but the less heat can be dissipated by the film. The heat must ultimately be dissipated through the exit window or other parts of the vacuum envelope. In doing so, thermal stresses will be produced which necessarily limit the power of the X-rays that can be generated in this manner.
In addition to the traditional uses of X-ray radiation sources, new applications have arisen in response to the threat of bio-terrorism or chemical agent terrorism. Chemical and gas methods for the remediation of hazards such as anthrax, ricin, or smallpox suffer a number of limitations, including hazards to human operators during their application, lingering hazards after they have been applied, limited effectiveness, long set-up and application times and destruction of electronic and other equipment in the treatment area. X-rays destroy biological agents through ionization. They can break chemical bonds and thus remediate chemical hazards. They can decontaminate biohazards in a matter of minutes or hours, compared to days and weeks with chemical and gas methods. X-rays have the further advantage of being able to penetrate objects or surfaces which may occlude hazardous material. However, sources of X-ray are needed which are compact, power efficient and do not suffer the limitations of prior art methods.